Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the pyrolysis furnace for further downstream processing, including quenching.
Historically, quenching effluent from a heavy feed cracking furnace has been technically challenging. Most modern heavy feed furnaces employ a two-stage quench, the first stage being a high pressure 10400 to 13900 kPa (1500-2000 psig) steam generator and the second stage utilizing direct oil quench injection. See, e.g., U.S. Pat. No. 3,647,907 to Sato et al., incorporated herein by reference. In the 1960s high pressure steam generating cracked gas coolers deployed as transfer line exchangers were found to be especially useful in cracking liquid feeds. The high steam pressure (8100 to 12200 kPa (80 to 120 bar)) and high tube wall temperatures (300° to 350° C.) limited the condensation of heavy hydrocarbons and attendant coke formation on tube surfaces.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking low cost heavy feedstock such as, by way of non-limiting examples, crude oil and atmospheric resid, also known as atmospheric pipestill bottoms. Crude oil and atmospheric resid contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile, heavy ends of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatiles can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Additionally, some naphthas are contaminated with crude oil during transport. Conventional pyrolysis furnaces do not have the flexibility to process resids, crudes, or many resid or crude contaminated gas oils or naphthas, which contain a large fraction of heavy non-volatile hydrocarbons.
Synthetic crude oils are wide boiling range hydrocarbon feeds that contain minimal amounts of non-volatile materials. Given the substantial absence of non-volatiles, e.g., resids (including asphaltenes), from synthetic crudes, they appear particularly suitable as feeds for cracking processes. However, conventional synthetic crudes that are hydrotreated blends of non resid containing virgin liquids from atmospheric or vacuum pipestills, combined with thermally cracked products, may exhibit difficulties in cracker operability. Such difficulties include low coil outlet temperatures, low conversion and high coking in the radiant and quench sections of pyrolysis furnaces.
U.S. Pat. No. 4,176,045 to Leftin et al., which is incorporated herein by reference, discloses production of C2 to C5 olefins by “steam pyrolysis, i.e., cracking” of normally liquid hydrocarbons while minimizing coke deposits on the interior surface of the furnace. More highly aromatic, higher coking petroleum derived feedstocks are blended with lower coking petroleum derived feedstocks to provide cracking feedstock.
Leftin, et al., “High-Severity Pyrolysis of Shale and Petroleum Gas Oil Mixtures,” Ind. Eng. Chem., Process Des. Dev., Vol. 25, No. 1, pp. 211-16, January, 1986, teach high-severity pyrolysis of narrow boiling range shale gas oil and petroleum-derived light gas oil mixtures to reduce coking rates as compared to shale gas oil alone as an alternative to hydrotreating shale gas oil prior to pyrolysis.
US 2005/0258073 to Oballa et al. discloses that “[a]n aromatics/naphthalene rich stream obtained by processing heavy gas oil derived from tar sands and cycle oils derived from cracking heavy gas oil may optionally be blended and subjected to a hydrogenation process and a ring opening reaction” in the presence of a catalyst “to produce paraffinic feedstocks for further chemical processing.”
Sharypov, V. I., et al., Fuel, Vol. 75, No. 7, pp. 791-94, discloses steam cracking coal-derived liquids with b.p.<350° C.
Gamidov, et al., “Pyrolysis of Coal-Derived Naphtha,” Azerb. Neftr. Khoz., (5) 37-40 (1989) Chem. Abstr. ABSTR. NO. 39538 V112 N6, teaches steam cracking a coal-derived hydrorefined naphtha provides reduced gaseous product yield (7-20%) than that of a straight-run petroleum naphtha, with the difference widening as severity of the process decreases. Ethylene yields were 3 to 7% higher for coal-derived naphtha under “medium high-severity conditions.”
When using synthetic crude oils as a feedstock to a cracker, it would be desirable to upgrade such feedstocks to improve cracker operability. Such improved feedstocks should provide higher coil outlet temperatures, higher conversion and reduced coking in the radiant and quench sections of pyrolysis furnaces.